Magnetic resonance imaging (MRI), nuclear magnetic resonance (NMR), and other magnetic resonance (MR) systems continue to become more sophisticated, powerful, precise, and complicated. MR systems rely on transmit coils to expose volumes to precisely controlled radio frequency (RF) energy. These RF transmit coils have also continued to become more sophisticated, powerful, and complicated. One recent advance involves using on-coil switched-mode current-source amplifiers.
MR involves the transmission of RF energy. RF energy used to produce MR may be transmitted by a coil. Resulting MR signals may also be received by a coil. Precisely controlling the generation of RF energy used to produce MR is desired to improve the resulting MR signals. However, conditions may change during application of RF energy. For example, circuits involved in producing RF energy may over-perform, under-perform, or fail completely. Yet sophisticated integration between numerous co-operating components may be required to produce and maintain precise conditions employed for producing MR.
The field of coil design using on-coil switched-mode amplifiers for parallel transmission is relatively new. For example, U.S. Pat. No. 7,671,595, which issued on Mar. 2, 2010 to one of these same inventors, presented an early on-coil switched-mode amplifier. U.S. Pat. No. 7,671,595 (the '595 patent) is entitled “On-coil Switched-mode Amplifier for Parallel Transmission in MRI” and describes an on-coil current-mode class-D (CMCD) amplifier that may be used to produce MRI transmission-coil excitations at desired RF frequencies. The on-coil CMCD amplifier is capable of performing within or proximate to the bore of the MRI magnet or within less than one wavelength of the RF signal produced by the transmit coil or at other positions or locations. Providing an on-coil amplifier allows digital control signals to be sent to the coil assembly, improving synchronization between the transmission-coils while reducing interference, cross talk, physical space requirements associated with cables, and heating normally associated with parallel transmission MRI systems.
Once this type of coil had been built, additional research using the new on-coil switched-mode amplifier revealed the need for further refinements including those presented in U.S. Pat. No. 8,294,464, which issued on Oct. 23, 2012 to one of these same inventors and in U.S. Pat. No. 8,294,465, which also issued on Oct. 23, 2012 to one of these same inventors. These patents describe improvements to the on-coil switched-mode amplifiers including the use of CMCD amplifiers, switched-mode pre-amplification, and amplitude modulation (AM) feedback for the on-coil switched-mode amplifiers. Once again, as these on-coil switched-mode amplifiers were built and used, further research revealed the need for further optimizations, particularly in telemetry systems.